Hedge trimmers, chain saws and angle grinders are usually provided with two handles to enable the user to control the tool more easily. However, the user will sometimes hold and operate the tool with one hand only and thereby expose himself to the risk of injury either to the hand not holding the tool or to another part of his body because he has insufficient control over the tool.
To minimise the risk of injury some tools, for example electrically-powered hedge trimmers are fitted with two series-connected control switches both of which have to be actuated before the tool is energised. In a hedge trimmer, one switch is built into the main handle of the trimmer and is usually in the form of a trigger-operated switch and will be referred to as a primary switch. Another switch to be referred to as a secondary switch is connected in series with the primary switch and is associated with the other handle of the hedge trimmer in such manner that the secondary switch can be operated only when the user grasps the other handle. Thus, the hedge trimmer will be energised only when both switches are operated and this occurs only when the user grasps the main handle with one hand and the secondary handle with his other hand. Removal of either hand from the respective handle will release the appropriate switch and the tool is de-energized.
It is known to place the secondary switch directly in the other handle and to link the switch to a movable part of the handle that is actuated when a user grasps the handle. In another known arrangement, the secondary switch is mounted in the body of the trimmer and a lever associated with the other handle is linked to the secondary switch. When the user grasps the handle, he actuates the lever which, in turn, operates the secondary switch via the linkage.
Mounting the switch directly in the other handle exposes the user to the risk of electrical shock and is therefore undesirable from the point of view of safety.
However, if the switch is mounted in the body of the trimmer, the risk to the user of electrical shock is small but interconnecting linkage between the switch and the lever has to be provided and accommodated and this increases both the cost and the weight of the tool.
Both known arrangements are also inherently unsatisfactory because there is the possibility that the movable parts required to operate the secondary switch may jam with the switch operated, in which case the tool will not be de-energised when the user removes his hand from the other handle. Such movable parts also require return springs which the user has to overcome to actuate the secondary switch and this gives rise to user fatigue.